1. Field of the Invention
The present invention relates to a control valve having first and second fluid chambers and a valve member which is biased in opposite directions by the fluid pressure in the first and second chambers. A viscosity sensitive restriction is provided in the fluid passageway which directs fluid flow to the first and second chambers. The viscosity sensitive restriction maintains substantially equal pressure in the first and second chambers upon initial pressurization thereof to prevent movement of the valve member from a neutral position upon initial pressurization of the first and second chambers.
2. Background of the Invention
Control valves for hydrostatic transmissions are well known in the art. Such valves are disclosed in the Hawk U.S. Pat. No. 3,646,762, the Knapp et al U.S. Pat. No. 3,972,189 and the Cornell et al U.S. Pat. No. 3,924,410. In the known control valves, it is generally required that physical symmetry of the hydraulic runs from the spool ends to the flapper and from the spool ends to the inlet passageway be present for equal volume and equal transient flow. It is also normal to eliminate dead end holes and pockets for air to collect to avoid a compressible fluid on either side of the spool valve. Adhering to these design criteria results in additional expense to the cost of the control valve.
In the prior art physical symmetry of the hydraulic runs was necessary to maintain the valve spool in its neutral position upon an initial pressurization where a low viscosity control fluid is utilized. If symmetry were not maintained, the pressures in the fluid chambers acting on the spool would not be balanced upon initial pressurization thereof and the spool would displace to a point where the servo means would be activated to move the swashplate from the initial neutral position due to transients in the fluid pressure acting on the spool.
The present invention attempts to solve the problems of the prior art while eliminating the need for symmetry in the hydraulic runs. A viscosity sensitive restriction is provided in the fluid passageway between the pressurized supply of control fluid and the fluid chambers which act on the spool valve. The viscosity sensitive restriction allows the transient startup pressures to be easily controlled. During a high viscosity, cold start, the viscosity sensitive restriction will allow the pressurized supply of control fluid flowing to each of the fluid chambers to rise at a rate which will not allow the upstream orifices to reach flow saturation; therefore, control fluid flow into one side of the spool valve and one chamber may be greater than into the other side of the spool member while still maintaining equal pressures between the chambers. Thus, the viscosity sensitive restriction gives the valve the ability to compensate for different compressible volumes due to an imbalance in physical layout. The colder the temperature, and the higher the viscosity of the control fluid, the lower the flow saturation of the upstream orifices and the greater the pressure drop across the viscosity sensitive restriction.